This invention relates to performing measurements on machine tools. It also relates to artifacts for use in such measurements.
Numerically controlled machine tools normally include position transducers such as scales and readheads. These indicate an X,Y,Z coordinate position for the machine's cutting tool at any given time. When manufacturing workpieces on a machine tool, it is a well known inspection technique to measure the workpiece on the machine tool itself, using a probe in the place of the usual cutting tool. The probe may for example be a touch trigger probe such as described in U.S. Pat. No. 4,153,998. Such inspection techniques make use of the machine's scales or other coordinate position transducers. Consequently, while useful for detecting if a workpiece is out of tolerance as the result of, say, wear of the cutting tool, this inspection technique is unable to detect inaccuracies of the workpiece caused by inaccuracies of the machine's position transducers.
Thermal effects are one cause of such inaccuracy in the machine's transducers. There are a number of random heat inputs during normal operation of the machine tool, for example from the normal operation of the machine's motor drives and lead screws, from machining operations, etc. Thus, thermal growth of various structural components of the machine tool is inevitable, both in the form of a general drift over a period of time and in the form of changes during the course of a given machining cycle for a given workpiece. The latter changes are also affected by the necessity to apply a deluge of coolant at various times during the machining cycle. Thermal expansion and contraction of the workpiece itself can also have a significant effect.
It is known to provide a pre-calibrated artifact on a machine tool, or on a coordinate measuring machine (which is a machine dedicated to inspection of workpieces rather than to their manufacture). Such an artifact may be a replica of the nominal shape and dimensions of the particular workpiece concerned, accurately manufactured and calibrated to a traceable standard. Alternatively, the artifact may be a universal artifact, having standard test features with dimensions which are accurately known to a traceable standard (e.g. the diameter of a bore or the length between opposing sides of the artifact). Such an artifact is placed upon the machine from time to time, and the machine performs test measurements on it. Comparing the test measurement results with the known, traceable dimensions indicates any inaccuracies of the measuring capabilities of the machine caused by thermal drift over a period of time, and future measurements made by the machine can be scaled accordingly in order to compensate for the inaccuracies.
However, such known techniques do not adequately compensate for all thermally induced errors which can occur on a machine tool. The present invention seeks to provide better techniques.